Histone deacetylases (HDACs) are one of the major classes of post-translational regulators and have been implicated in pro-growth, anti-apoptotic, and anti-differentiation roles in various cancer types. As the key enzymatic components of multiprotein complexes, histone deacetylases (HDACs) are responsible for deacetylation of lysine residues in histone and nonhistone protein substrates. Recently, HDAC inhibitors have been found to arrest growth and induce apoptosis in several types of cancer cells, including colon cancer cells, T-cell lymphoma cells, and erythroleukemic cells. Given that apoptosis is a crucial factor for cancer progression, HDAC inhibitors are promising reagents for cancer therapy as effective inducers of apoptosis (Koyama, Y., et al., Blood 2000, 96, 1490-1495).
HDAC proteins comprise a family of 18 members in humans with homologies to yeast HDACs, Rpd3, Hda1, and Sir2. Based on their sequence similarity, cellular localization tendencies, tissue expression patterns, and enzymatic mechanisms, the HDACs can thus be divided into four classes. The class I HDACs (HDACs 1, 2, 3, and 8), homologous to Rpd3, localize primarily in the nucleus and appear to be ubiquitously expressed in most tissues. The class II HDACs (HDACs 4, 5, 6, 7, 9, 10), homologous to Hda1, are able to shuttle between the nucleus and the cytoplasm depending on a variety of regulatory signals and cellular state, and are expressed in a more limited number of cell types. These HDACs can be further subdivided into class IIa (HDACs 4, 5, 7, 9), and class IIb (HDACs 6, 10). HDAC11 is the sole member of class IV histone deacetylase. Class I, II, and IV HDACs are all zinc-dependent deacetylases. In contrast, the class III HDACs, homologous to Sir2, are NAD+-dependent deacetylases that are mechanistically distinct from the class I and II HDACs and are not inhibited by classical HDAC inhibitors such as trichostatin A, trapoxin B, or MS-275.
Given their association with cancer formation, class I and II HDAC proteins have emerged as attractive targets for anticancer therapy. The class I HDACs in particular have been closely associated with anti-proliferative effects against tumor cells. For example, pharmacological inhibition of HDACs 1-3 leads to induction of the cyclin-dependent kinase inhibitor p21 and concomitant cell cycle arrest. Several HDAC inhibitor (HDACi) drugs are in various stages of clinical trials, with SAHA (suberoylanilide hydroxamic acid, Vorinostat) and Romidepsin (FK228) gaining FDA approval in 2006 and 2009 respectively, for the treatment of cutaneous T-cell lymphoma (CTCL). Recently, the expression of HDAC8 (and not any other HDAC isoforms) was shown to significantly and independently correlate with the disease stage and poor survival of neuroblastoma (NB), which is a neoplasm of the peripheral autonomic nervous system that represents the second most common malignancy of childhood. Furthermore, knockdown of HDAC8 by siRNA led to NB cell differentiation and inhibited cell growth while its overexpression blocked retinoic acid-induced NB differentiation (Clinical Cancer Research 2009, 15, 91-99). HDAC8 is therefore a potential drug target for the differentiation therapy of minimal residual disease in NB. In addition, a possible correlation between HDAC8 and acute myeloid leukemia (AML) has also been suggested (Bioorg. Med. Chem. Lett. 2007, 17, 2874).
Unlike class I HDACs which are predominantly nuclear enzymes, class IIa enzymes shuttle between the nucleus and cytoplasm, and are known to associate with the HDAC3/SMRT/N-CoR complex and MEF2 and as such have important roles in regulating muscle cell gene expression (reviewed in Oncogene 2007, 26, 5450-5467) and the immune response (Biochemical Pharmacology 2007, 74, 465-476). The IIb subclass enzymes uniquely feature two deacetylase domains, and are primarily cytoplasmic. Significantly, HDAC6 operates on a variety of substrates other than histone proteins, and is involved in processing Lys40 of the mitotic spindle protein α-tubulin. HDAC6 also has a dynein motor binding domain to enable HDAC6 to shuttle cargo along the microtubule, and a zinc finger ubiquitin-binding domain at the C-terminus. Through its ubiquitin-binding activity, HDAC6 is able to mediate the recruitment of autophagic material to aggresomes for degradation, thus decreasing the cytotoxic effects of these aggregates (Cell 2003, 115, 727-738). Inhibition of HDAC6 activity by the specific inhibitor, tubacin, can increase accumulation of acetylated α-tubulin and inhibit cell motility without affecting microtubule stability per se (J. Am. Chem. Soc. 2003, 125, 5586-5587, Proc. Nat. Acad. Sci. USA 2003, 4389-4394).
Multiple myeloma (MM) is a plasma cell malignancy characterized by complex heterogeneous cytogenetic abnormalities and infiltration of malignant cells into the bone marrow, leading to bone disease, hypercalcemia, cytopenia, renal dysfunction, hyperviscosity and peripheral neuropathy. Standard proteasome inhibitor-based therapies have achieved remarkable response rates in MM, however combination therapies with new targeted drugs are still needed due to the development of drug resistance and poor long-term survival. It was recently demonstrated that concomitant proteasome and HDAC6 inhibition can lead to synergistic anti-proliferative effects in MM cells, most likely due to the role of HDAC6 in mediating aggresome function and the ensuing misfolded protein stress that develops as a result of dual proteasome/aggresome inhibition (Proc. Nat. Acad. Sci. USA 2005, 102, 8567-8572). HDAC6 is therefore an attractive novel target for the development of new MM combination therapies.
The compounds according to this invention are inhibitors of HDAC6 or HDAC8 and therefore show anti-proliferative and differentiation-inducing activities, which result in inhibition of tumor cell proliferation and induction of apoptosis. Pan HDAC inhibitors have broad spectrum preclinical activity against a wide range of cancer types, yet also possess non-specific cytotoxicity which may limit their clinical application. In contrast, HDAC inhibitors targeted toward specific isoforms, especially HDAC6 and HDAC8, typically show lower non-specific cytotoxicity and can be suitable for the treatment of certain cancer subtypes. The compounds of the present invention show enhanced selectivity toward HDAC6 or HDAC8 compared with the pan HDAC inhibitor SAHA, as assessed by both enzymatic and in-cell assays.
Based on different zinc binding groups, four major classes of HDAC inhibitors have been extensively described in the literature: (1) hydroxamic acids; (2) ortho-aminoanilides; (3) thiols or their prodrugs; (4) carboxylic acids and their analogues (reviewed in J. Med. Chem. 2003, 46, 5097-5116). In general, the hydroxamic acids such as SAHA, LBH589, PXD101, JNJ26481585 and ITF2357 display broad inhibitory activity against most HDAC isoforms in the submicromolar range (J. Med. Chem. 2007, 50, 4405). On the other hand, the ortho-aminoanilides exemplified by MS275 and its aryl substituted analog show high potency and class I activity confined primarily to the HDAC 1, 2, 3 subtypes. The thiol prodrug FK228 (depsipeptide/Romidepsin) also has been reported to have similar class I selectivity, although the drug's developer, Gloucester pharmaceuticals, has claimed that the molecule is a pan-HDAC inhibitor (Mitchell Keegan, Discovery On Target HDAC Inhibitor Conference 2007). In contrast, the fatty acid class are the least potent of the HDAC inhibitors, with enzyme inhibitory values in the high micromolar ranges.
Limited reports confined to the realm of hydroxamic acid-based molecules have been published describing compounds with HDAC6 and/or HDAC8 selectivity. Tubacin is the prototype HDAC6 selective inhibitor with a bulky capping group contacting the rim region of HDAC6. Kozikowski et al. have described potent HDAC6-selective triazolylphenyl capped hydroxamates and related phenylisoxazole capped hydroxamate inhibitors with greater than 50 fold selectivity over HDAC1 and HDAC3 (J. Med. Chem. 2008, 51, 3437 and J. Med. Chem. 2008, 51, 4370). In all instances, the inhibitors have rigid and bulky capping groups as selectivity elements and those capping groups are linked with zinc binding hydroxamic acids through flexible aliphatic chains. In a different approach, Envivo Pharmaceuticals disclosed 1,2,3,4-tetrahydroisoquinoline hydroxamates for potential treatment of neurodegenerative diseases (WO2005/108367), but their HDAC isoform selectivity has yet to be clarified. Most recently, Smil et. al. from MethylGene Inc. reported chiral 3,4-dihydroquinoxalin-2(1H)-one and piperazine-2,5-dione aryl hydroxamates with selectivity (up to 40-fold) for human HDAC6 over other class I/IIa HDACs.